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Driveability Corner

OBD II Mode 6 is unfamiliar territory for many technicians. Valuable (but occasionally cryptic) diagnostic information can be found there, if you know how to look for it.

Mode 6? What's Mode 6? OBD II is divided into modes as defined by the EPA and SAE. These modes and their operations are defined in SAE On-Board Diagnostics for Light and Medium Duty Vehicles Standards Manual. This manual can be purchased from the Service Technicians Society (STS) at their website, www.sae.sts.org.

What does "Test results for continuously and noncontinuously monitored systems" mean? As I've explained in previous columns, the "monitors" in OBD II are the on-board tests performed to confirm that a vehicle has been adequately checked for problems so it can be emissions-tested (an OBD test). Mode 6 shows the results for the tests run to set the monitors to "ready or complete." Therefore, Mode 6 may give you insight into problems that have not yet affected monitors or tripped a code. This is why Mode 6 has high value in some situations.

Mode 6 functionality is defined by the vehicle manufacturers and therefore is not the same from one make to another. This means you'll have to check to see what's reported for each make and model and determine its value. Mode 6 is also not supported by all scan tools, and may be unclearly defined by some tools that do support it.

Let's look at a misfire diagnostic example. GM has great misfire diagnostics in its enhanced (non-OBD II generic data) with the misfire counter parameter (see my scan tool misfire diagnostics column in the April 2001 issue of Motor). Ford does not support a historical misfire counter like GM does. But Ford does monitor the injector circuit for shorts and opens in the parametric data (see Fig. 1 on page 16). This is a Mercury Grand Marquis from which we disconnected the No. 2 injector. Note parameter 25 showing a fault with this injector. This is live data; therefore, the fault will appear only when it's present, unlike a flag that trips when a fault occurs and stays until reset. It's important to have a tool capable of recording data to catch an intermittent fault of this type.

The injector fault data is great for diagnosing an injector electrical problem, but what about other misfire problems-including ignition problems, plugged injectors, mechanical problems, vacuum leaks and all those other pesky misfire issues? Sure, you're in pretty good shape if the PCM stores a misfire code, but how about an early warning? Fig. 2 at right shows the Mode 6 data from the same vehicle. This vehicle had not yet set a code and may not for some time (perhaps two trips).

The Mode 6 data in Fig. 2 not only identifies the existence of the misfire, it also correctly identifies which cylinder is misfiring. So, is Mode 6 data dynamic or a flag that gets set? Testing on Fords indicates that it's dynamic data, which means that Mode 6 shows the misfire only while it's happening, and won't help to identify a cylinder that was misfiring previously.

Mode 6 data may not always be clear, and it may be reported in undefined units. For example, let's say the bank 1 sensor 1 oxygen sensor shows a value of 25, and the failure limit is 525. Although we don't know the unit or the test parameters, it appears the sensor is probably okay. The same vehicle with a driveability problem and an O2 value of 500 should clue you in to the fact that more O2 testing is in order.

Checking the O2 Mode 6 data is important when a catalyst efficiency code is set. Catalyst efficiency is determined by the O2 sensors, and sluggish sensors may falsely indicate a catalyst problem when none exists. Anytime a code is set, if possible, check the related Mode 6 parameters for supporting or contradictory information.

Want more Ford Mode 6 info? Check out the Ford reference data on i-ATN. Here are quick links to two sections:

Go to the end of the document for the Mode 6 info. You may want to spend some time exploring this section, because there's a lot of good information to be found. These are just two links.

Is Mode 6 data clear on every car? No. Will Mode 6 help every time? No. It's worth the effort to check it out, though. Ask your scan tool manufacturer how to access Mode 6 data on its equipment. This data might save you some serious time and money on your next diagnosis.
It's just another tool in the diagnostic arsenal. Take a look; you might like it!